Despite advances in the production of early-setting, high-strength cements, the development of Portland-type cements with even greater compressive strengths and higher rates of strength development would be highly beneficial to the cement and the construction industries. For example, producing pre-cast, pre-stressed, concrete products with compressive strengths on the order of 4000-5000 psi often requires at least three hours or more.
Additionally, the construction and repair of highways, bridges, and freeway overpasses requires many days and even weeks of curing time before these structures set to sufficient compressive strengths to support their anticipated loads so that they may be utilized as designed. The resulting delays are a significant component of construction costs, which are compounded by increased transportation costs and shipping delays while critical transportation corridors are shut down waiting for concrete to harden.
Moreover, in the construction of concrete buildings, in which the cement matrix is cast into forms, days of curing time are necessary to allow the cement to develop sufficient strength before the forms may be removed. Such delays result in lost revenues for property owners, as well as inconvenience and storage costs for industrial tenants. Furthermore, setting rates of Portland-type cements can be affected by temperature. An early setting, ultra-high strength cement with a lower heat of hydration would make the production of large, complex superstructures possible at low ambient temperatures and would be an even greater contribution to the construction industry.
Many cement-based products are also mass manufactured in high-throughput factory situations. The time that it takes for cement to cure or set completely adds to the cost and difficulty of manufacturing. In particular, fiber cement products are manufactured in this way, by processes such as the Hatschek process, the Mazza process, or by extrusion. When a fiber cement product is formed in its “green” state, it typically must set for 8 to 12 hours (“precure”) before additional processing or handling can occur. Precuring requires warehouse space as green products accumulate while setting, which adds to the cost of the manufacturing process. Consequently, there is a strong business motivation to reduce the curing time of cement, and in particular, fiber cement materials. Furthermore, methods of reducing the curing time at a substantially lower cost than conventional quick-setting cement formulations, such as high alumina cement, are very desirable.
The best-known and most commonly used accelerant for setting cement is calcium chloride. However, it is widely known in the art that calcium chloride is incompatible with steel-reinforced cements due to its tendency to corrode the steel reinforcement over time in the presence of water and oxygen. Calcium chloride is also sometimes used as an accelerant in fiber cement manufacturing to obtain green strength quickly; however, buildup of chloride ions can also cause corrosion problems in downstream manufacturing processes such as autoclaves. Accordingly, an accelerant for setting concrete and cement composites that does not cause corrosion is needed.
Besides calcium chloride, other accelerants, such as alkaline bases (sodium hydroxide, potassium hydroxide, ammonia, sodium or potassium carbonate, and sodium or potassium silicate), alkaline or alkaline earth metal nitrates, nitrites or carboxylates are all chloride-free with reduced corrosiveness compared to calcium chloride. However, these materials must still be added separately to concrete or cement composite formulations and thus require their own storage vessels, metering apparatus, and mixing/blending processes, either manual or automated, to incorporate them into a composite. On the other hand, it is well known that low-density modifiers for concrete, such as perlite or ceramic microspheres, do not accelerate cement hydration. A single material that provides both an accelerated curing effect and some other desirable function, such as density reduction, in a concrete or cement composite would save considerable time and equipment capital.
There are several well-known organic materials used in cementitous composites and concrete applications. These so called admixtures include cellulose ethers for rheology modification, lignosulfonate or naphthalene sulfonates as water reducing agents and superplasticisers, polymer latexes or emulsions for modifying water absorption properties or improving flexibility, as well as other organic admixtures well known in the art. One typical effect of these admixtures is retarding the setting time of the cement. Often, an accelerant is used to counteract this retarding effect. Accelerants known in the art are corrosive and/or expensive relative to ordinary Portland cement and can add significant cost to a concrete or cement composite formulation.
LeTure et al., in U.S. Pat. No. 5,709,743 disclose a calcium silicate hydrate based accelerant that does not cause corrosion and is as effective as calcium chloride. This material is prepared by hydrating Portland cement to form a calcium silicate hydrate (CSH) material. This CSH material is then finely ground into “crystallization seeds” which accelerate the cure of cement cubes when added in an aqueous suspension to cement. Grinding or milling the calcium silicate particles to a specific particle size (as determined by sedimentation volume) is a crux of the invention described in U.S. Pat. No. 5,709,743. Grinding or milling such particles inherently increases their packing density and thus CSH particles made according to the disclosure of LeTure et al. are unsuitable as a low-density additive.
Another drawback of using the CSH accelerant of LeTure et al. is that it must be added as aqueous slurry. This is a disadvantage for cement composite mixes, such as extrudable cement composites in which the water content is carefully managed to ensure consistent product quality.
One way to achieve green strength quickly in concrete is to use “high early strength” cement, which is ground to a finer particle size and has greater surface area (450 to 600 m2/kg, Blaine method) than standard Portland cement (300 to 400 m2/kg). These fine cements are not suitable for use in the Hatschek process typically used to make fiber cement composites because the fine cement is not retained in the composite during the slurry dewatering process.
In fiber cement manufacture, low-density additives often reduce the strength-to-weight ratios of the fiber cement. Since building materials must meet specific strength criteria, this effect limits the amount of low-density material that can be used in a fiber cement composite.
When a green fiber cement paste is extruded under pressure through a die, the green uncured fiber cement paste tends to “springback” after leaving the extrusion die. The degree of springback is a function of the cement paste rheology, processing history, fiber content and type, air entrainment, and other factors well known in the art. One way to reduce springback is to increase the length of the die, allowing the green fiber cement paste more time to “relax” in its compressed state. However, longer dies are more expensive than shorter dies, and longer dies tend to increase the pressure required to extrude the fiber cement paste. High-pressure extrusion can damage fragile low-density aggregates used in the paste, so a low cost material additive would be a better solution.
Forming concrete underwater, either for repair of existing structures or the building of new structures, often results in the “washout” of fines from the freshly mixed concrete and/or in segregation of the concrete mix. Consequently, most concrete mixes used for underwater placement include a high viscosity cellulose ether material as an anti-washout admixture. However, where large amounts of concrete need to be placed in areas with severe water flow and/or water pressure, such as repair of dams and back filling the annular void behind tunnel linings, the cellulose based anti-washout admixtures can be ineffective in completely preventing washout and segregation, mainly due to its extended delay in cement hydration.